When you run FlashBack Pro for the first time, it will automatically download a trial license. If you see the watermark, that trial license is stilll being used and needs to be replaced with your full license.
When you start FlashBack while it is using a trial license, a pop-up will appear which tells you how many days are remaining on your trial. Click the ‘Enter full license key…’ option on that pop-up and enter the new licence key that you will have received by email.
(If you simply upgrade the trial license, that change will automatically be applied when you next start FlashBack so there is no need to enter a license key)